The present invention relates to inorganic cementitious structures such as concrete beams or gypsum castings and, more particularly, to structures of this type that are reinforced by preformed shapes comprising carbon or graphite fibers.
Portland cement concrete is commonly reinforced by steel rods which provide stiffness, impact strength and load bearing characteristics. It is also known, as in U.S. Pat. No. 4,199,366, to add various natural and synthetic fibers to various cementitious substrates. In concrete mixtures before they are poured, natural fibers such as cotton, jute, sisal, hemp and wool have been mixed into the concrete, as have such synthetic fibers as rayon, nylon, polyesters, polyvinyl alcohol, polyacrylonitrile (PAN), polyvinyl chloride, polyethylenes, and miscellaneous fibers such as glass, carbon, and metallic carbides. It is also known to use various fiber reinforcements for other types of cementitious structures such as gypsum castings based on the hemi-hydrates of calcium sulphates, or magnesium-oxide, chloride, lime and other inorganic cementitious mixtures.
Most of the natural and synthetic fibers mentioned above will lose some of their reinforcing qualities in concrete over the years as they are exposed to the alkaline environment of the free-lime appearing in Portland cement concrete. Even glass fibers suffer degradation in time during their utilization in Portland cement concrete. As for reinforcing steel rods, they must be well encapsulated within a Portland cement concrete structure to minimize the effects of air and moisture which cause corrosion of the steel. The products of steel corrosion, being of lower density than steel, have a volumetric expansion on forming which may crack the concrete, which imposes a design limitation in that steel reinforcements cannot be positioned too close to the surface since surface cracks will appear and the quality of the concrete structure will deteriorate.
While the randomly oriented fibers of the prior art contribute to toughness and fatigue resistance of the reinforced structure, fairly large quantities thereof must be used, typically in excess of 3% by weight, in order to obtain substantial improvements. By the time such volumes of these fibers are added and randomly dispersed by prevailing mechanical mixers, the slurry is thickened to such an extent that more energy is required and, also, the probability of air entrapment is greatly increased. Thus, while the addition of fibers in this fashion may enhance the concrete toughness, there is also the undesirable consequence from a long range standpoint of loss in environmental resistance due to the presence of occluded air pockets.